1. Field of the Invention
The present invention relates to a guided-wave circuit for use in the field of optical communication or optical information processing. More specifically, the invention concerns a guided-wave circuit with an optical characteristics adjusting plate which uses the optical characteristics adjusting plate to adjust errors inevitably occurring between the optical characteristic values on the input side and the optical characteristic values on the output side of a plurality of optical waveguides formed on the chip of the guided-wave circuit; a method for producing the guided-wave circuit; and an apparatus for producing the optical characteristics adjusting plate.
Examples of the guided-wave circuit are double beam interferometers and multiple beam interferometers composed of optical waveguides formed on a flat plane. Examples of the optical characteristics are the phase and amplitude of light propagating through the plurality of optical waveguides, as well as the birefringence of the optical waveguides.
2. Description of the Prior Art
In recent years, energetic studies have been conducted on planar lightwave circuits (PLC's) constructed, for example, by silica-based optical waveguides formed on a silicon substrate. These circuits employ double beam interferometers or multiple beam interferometers such as Mach-Zehnder interferometers or arrayed-waveguide grating multi/demultiplexers to achieve the switching or multiplexing/demultiplexing function.
Thermo-optic switches utilizing the Mach-Zehnder interferometer are described in detail in Okuno et al., "Silica-based Thermo-optic Switches", NTT R&D, vol. 143, No. 11, pp. 1289-1298, November 1994. This type of switch realizes the switching function by thermally controlling the difference between the optical path lengths of the two arm waveguides by means of thin-film heaters formed on the surface of the waveguides.
FIG. 1 shows the outline of the configuration of the PLC, and FIG. 2 shows an enlarged sectional view taken on line II--II of FIG. 1.
In FIG. 1, the numerals 101-a and 102-a represent input ports, 103 is a silicon substrate, 104-1 is the first arm waveguide, 104-2 is the second arm waveguide, 105 is a thin-film heater, 109 is a cladding layer, and 114 is a core layer.
The characteristics of an example of a 2.times.2 thermo-optic switch produced are shown in FIG. 3, in which the horizontal axis represents electric power applied to the thin-film heater, while the vertical axis represents the through-port (101-a.fwdarw.101-b) transmittance of light. The transmittance varies with the electric power applied to the thin-film heater. By alternating between P1 and P2 as the electric power at certain time intervals, this circuit acts as a 2.times.2 optical switch.
The two arm waveguides 104-1 and 104-2 shown in FIG. 1 are designed to have the same length. Thus, when the through-port transmittance is minimal, no electric power should be applied. In other words, P1 should be zero.
However, because of the error in the production of the waveguide, an optical path length difference of the order of 0.1 .mu.m occurs between the lengths of the two arm waveguides 104-1 and 104-2, so that P1 is not zero. This optical path length error of 0.1 .mu.m is about 10% of the optical wavelength, and thus the value of P1 is not negligible compared with the switch power (P2-P1). Furthermore, the optical path length error of the order of 0.1 .mu.m is an error corresponding to about 10.sup.-5 for the arm waveguides 104-1 and 104-2 measuring about 10 mm. Markedly decreasing this value is difficult with the manufacturing technology.
The power P1 increases the electric consumption of the switch, and should preferably be zero in value.
With an arrayed-waveguide grating wavelength multi/demultiplexer, the optical wavelength multi/demultiplexing effect is achieved by interferences of a plurality of optical beams propagating through about 30 to 100 arm waveguides arranged in parallel and different in the optical path length from one another by n.times..DELTA.L where n denotes the effective refractive index of the waveguide, and .DELTA.L represents a value of about 10 to 100 .mu.m. The details are described in H. Takahashi et al., "Arrayed Waveguide Grating for Wavelength Division Multi/demultiplexer with Nanometer Resolution," Electron. Lett., vol. 26, No. 2, pp. 87-88, 1990.
FIG. 4 shows the outline of the circuit configuration of this multi/demultiplexer. In FIG. 4, the numeral 110 represents input waveguides, 111 output waveguides, 112 slab waveguides, 113 arrayed waveguides, and 103 a silicon substrate.
FIG. 5 shows the wavelength-transmission characteristics of transmitted light from the central input port to the central output port of the arrayed waveguide grating wavelength division multi/demultiplexer shown in FIG. 4. As shown in FIG. 5, only particular wavelengths are transmitted from the central input port to the central output port, while light of other wavelengths is blocked.
For the moment, the crosstalk, expressed as the ratio of the transmittance for the blocked wavelengths to the transmittance for the transmitted wavelengths, is about -30 dB.
Decreasing this crosstalk is a very important task of the wavelength division multi/demultiplexing function. The first cause of the crosstalk being restricted to about -30 dB is that the optical path length difference n.times..DELTA.L set in the arrayed-waveguide fluctuates on the order of 0.1 .mu.m owing to the manufacturing error, thereby arousing errors in the phases of light passing through the respective arrayed waveguides.
The second cause for the restriction is that the amplitude of transmitted light from each path after distribution to the respective arrayed waveguides at the branching portion and recombination at the multiplexing portion deviates from the designed value because of the nonuniformity of the waveguide loss; namely, an amplitude error occurs.
With an optical circuit in which the waveguides account for a large area, an arrayed waveguide grating with a narrow channel spacing is provided. Moreover, variations in birefringence occur in the optical waveguides constituting such an optical circuit. Thus, the phase distribution differs depending on the polarization of the optical waveguide, so that the polarization dependency of the characteristics arises.
As described above, the optical path length error on the order of 0.1 .mu.m during the manufacture of the PLC brings about deterioration of the characteristics of a double beam or multiple beam interferometer. If this optical path length error can be adjusted, the characteristics of the interferometer can be improved.
Another error which occurs during the manufacture of the PLC is the deviation, from the designed value, of the amplitude of transmitted light from each path after distribution to the plurality of channel waveguides at the branching portion and recombination at the multiplexing portion. This deviation also causes deterioration of the characteristics of a double beam or multiple beam interferometer. If this deviation in amplitude from the designed value can be adjusted, the characteristics of the interferometer can be improved. By adjusting the amplitude and phase characteristics to the desired values, moreover, it becomes possible to add the functions of flattening the wavelength pass band and controlling the dispersion.
Moreover in an optical circuit in which the waveguides occupy a large area, the polarization dependency of the characteristics occur owing to variations in birefringence of the optical waveguides constituting the optical circuit. If the variations in birefringence can be controlled, a high performance optical circuit can be prepared without relying on the polarization state.